In recent years, requirement for high-strengthening of a steel sheet used for a vehicle and so on becomes high, and a high-strength steel sheet whose tensile maximum stress is 900 MPa or more comes to be used. On the other hand, it is required to show excellent forming workability at a forming work time such as presswork. However, improvement in the strength easy to incur deterioration in the forming workability, and it is difficult to satisfy both requirements.
Accordingly, in recent years, a steel sheet using bake hardenability (BH-ability) by a coating/baking process (bake hardening process) after the forming work has been developed so that workability such as ductility and hole expandability and high-strengthening are both enabled.
Here, the bake hardening is a phenomenon in which C (solid-solution C) and N (solid-solution N ) remaining in a steel sheet in a solid-solution state diffuse to dislocations during a baking process (normally heated up to approximately 170° C., then kept for several dozen minutes) after coating, the dislocations are fixed, and thereby, a yield strength increases. The increased amount of the yield strength is a coated bake hardening amount (BH amount), and the BH amount is generally known to increase by increasing a solid-solution C amount or a solid-solution N amount.
In Patent Literature 1, a cold-rolled steel sheet is disclosed in which a hard structure made up of bainite and martensite is a main structure thereof, and a high bake hardening amount is secured by limiting a fraction of ferrite into 5% or less.
Besides, in Patent Literature 2, a high-strength cold-rolled steel sheet is disclosed in which bainite is a main structure thereof, a hardness ratio between bainite and ferrite is made small, and dispersion of hardness in each structure is made small to thereby improve the bake hardenability, the ductility, and the hole expandability.
Besides, in Patent Literature 3, a method is disclosed in which a steel sheet is made to be one containing tempered martensite and/or tempered bainite obtained by performing annealing for a hot-rolled steel sheet without performing cold-rolling, or performing the annealing for two times after the cold-rolling to thereby improve the workability and the bake hardenability.
Besides, in each of Patent Literatures 4, 5 and 6, an art improving the bake hardenability adding a lot of N is disclosed